Title clearly says it here, I'd figure with it being only a good 9 months away before we officially get proper news, it'd be a good idea to get this topic up and out of the way which can be used to discuss once it officially drops in July 2017 (feel free to make your own as this was exactly why I made this thread).
Cast members predictions: (note this only applies to the main characters as we know, sans Jack-Jack).
Craig T. Nelson- Bob Parr/Mr. Incredible (a no-brainer, Disney Infinity proved that he can still get the voice down regardless of age, unless they have his backup VA on hand in case Nelson passes on like Pena did (he's 71 pushing on 72).
Holly Hunter- Helen Parr/Elastgirl/Mrs. Incredible (I think that interview with her promoting Batman V Superman more or less sealed the deal on her coming back, the reason behind me putting Mrs. Incredible as a another superhero name is because I think they may drop the name Elastigirl and just go with her other name to avoid another lawsuit from DC comics).
Sarah Vowell- Violet Parr/Invisigirl (another no-brainer, there's no way I can see Vowell getting replaced anytime soon (and she's rumored to be back according to IDMb), she'll obviously be named Invisigirl to market the character a little better (plus it's on her NSA database profile), although I fear this could anger Marvel in a similar vein to them angering DC over the Elastigirl name (as the name Invisigirl is similar to another superhero name, The Invisible Woman from the FF4 (whom the Incredibles copied) I'd love to see her named Miss Disappear as a drawback.)
Raymond Ochoa- Dashiell Parr/Dash (rumored to be back according to IDMb (the same site that also rumored Vowell's return), granted his voice is getting deeper by the day, if he gets replaced Hayden Rolence should replace him.
Samuel L. Jackson- Lucius Best/Frozone (too easy to predict as he explained it way back in 2014 that he wanted in and was likely to be the first one cast).
Jason Lee- Syndrome (I mostly see him in flashbacks via the unused scenes from the first film, granted seeing him back alive and well would be quite a shocker indeed (as Bird did talked about unique plot twists, him returning would be one of them, abit a very predictable twist as we seen in most superhero films), but I highly doubt it after what he went through (unless we get a sibling of his or the actual mastermind behind his rise to power to show up in his place, plus I think most official canon clearly states that he indeed perished when his jet plane blew up).
Bob/Mr. Incredible- Can't really see him changing a whole lot (aside from maybe receding his already short hair further, depending on how far in time we go).
Helen/Mrs. Incredible- Same deal as Bob (although I think she'll get skinnier as the plot thickens, basically making her workout in the same vein as her husband did in the first film she was pretty much out of shape in the first film, but not as bad as Bob's.)
Violet Parr- I expect her design to remain exactly as she was in the first film, but maybe a bit taller while keeping her weight the same as before (I can see her reaching her mom's shoulders or at least her elbows), I also expect her iconic long hair to be retained (with maybe a couple of different styles for certain scenes, but we all want that shy/goth hairstyle back in some form as that is what made Violet in the first place), cutting her hairstyle or remaking it like Riley's hairstyle was in Inside Out, Violet loses the one thing that gives her character the charm it needs (granted with the improved tech with the new Renderman software that was used in Finding Dory I don't see Violet's long hairstyle being a major issue unlike the first film (the first film would have made sense for a short haired Violet, then grow it out for the sequel to show her growth).
Dash Parr- Same deal as Violet, not much you can really do to change his looks aside from a small height increase (unless we get a large time-skip), but I highly doubt it with the direction Bird's going, granted little boys do grow up very fast compared to girls.
Jack-Jack Parr- He'll obviously get the most changes out of the entire cast to move him into a major role, seeing as it's looking more likely he'll be the main character or at least the main focus of the sequel.
Characters who I can see returning/axed:
Mirage- Granted this will be very hard to pull off with her original VA now deceased (RIP Elizabeth Pena), finding a replacement will likely have to fall back on Cars 3 or Coco (as both films are expected to have Latina speaking female actresses in them), Mirage has got to be in this sequel to give her character closure, along with paying respects to Elizabeth Pena who would no doubt get a tribute to her memory (I'd love it for it to be in a small scene from the first film to be played from Mirage's last appearance before the Parrs head back to Metroville to fight the last Omnidroid).
Tony Rydinger- Going into this film, I clearly thought that Tony would be an obvious shoo-in choice to return, but after taking a small look at TVtropes and viewing how they picture him in general, I think he's going to get axed as he was more or less there just to show Violet's growth in the first film (him and Violet may have had a falling out regarding superheroics/Dash revealing her superhero identity to him which causes him to break up with Violet and moves away which would be a good way to bring back Violet's shy self for a good portion of the film as a reason for her being depressed over her breakup with Tony.)
Kari McKeen- I can see her returning for comedic relief, plus we barely know a thing or two about her, plus Violet needs a friend whom she can rely on (especially if Tony gets dropped), even if Kari's not up to par (pun intended), plus if Kari were to get axed Violet will be all lonely and that's not a good way to start off Violet's character right.
Edna Mode- No capes enough said, she's pretty much back, I hope we get a bit of a backstory on her past a bit.
Rick Dicker- I can see him back, we could use more of this guy, plus a backstory on the NSA and his history with Bob and Helen would be fun to explore.
Not really sure who'd make a good new character (we'll obviously get a new lead villain to replace Syndrome for sure), I'll leave that up to you fellows, granted I can see the comics becoming canon (abit a bit retconned to fit the actual story-line from the first film better, although I know that'll anger a lot of people as dozens of fans considers the comic non-canon), I can easily see a new boy-toy for Violet should Tony get axed (whether it'd be a retconned Xander or some new guy).
As much as I hate to admit it, Jack-Jack will more than likely be our main character as he has the least amount of development compared to the rest of the family (there's clearly too much evidence to support this), plus I can see him dropping his powers over fears he may actually kill people with them (including his own friends and family), this would be a good way to show off everyone else's soft sides (this could also resonate well with Violet's story as she would likely blame her powers for her breakup with Tony), I except at least a 2-3 year time skip (4-5 could be doable (Violet on the verge of graduating high school could easily pull of the idea of the parents letting her go her own way, although a 2-3 year gap could pull that off just as well as it could set up a third installment).
Feel free to comment on your opinions or ideas and predictions.